This invention relates to pyrotechnic devices, and more particularly to improvements in initiators for use in automotive air bag systems.
A typical air bag initiator is an electroexplosive device used to initiate a sodium azide-based propellant charge for inflation of the air bag. However, the technology of the air bag inflation system is changing and new types of inflator technology such as solid organic propellants, stored gas hybrid, liquid propellant and hydrogen oxygen flammable gas systems are being developed. As a result the requirements on the initiator system are changing to require more structural integrity of the initiator body as well as new technology which allows the initiator to be hermetically sealed into the inflator cavity under high pressure for the life of the unit. The initiator generally comprises an ignition charge which is in direct contact with a bridgewire. The bridgewire is connected between two contacts which are connectable to a firing circuit. When the bridgewire is electrically heated, it ignites the ignition charge. An output charge or "booster" charge, ignitable by the ignition charge, may be used to effect quick ignition of the sodium azide-based propellant without damaging the propellant grains.
Electrical connections are typically made to the firing circuit respectively from the initiator case and from a central conductor which is in coaxial relationship with the case and extends into the case through an insulator. Devices using this form of electrical connection are known as "coaxial" devices. Alternatively, electrical connections can be made through a pair of parallel, spaced conductors both of which extend into the interior of the initiator case through an insulator. Devices having this form of electrical connection are known as "twinaxial" devices.
One of the problems encountered in the manufacture, installation and use of airbag initiators is the problem of protection against accidental ignition resulting from static electricity. A substantial static charge can accumulate on a worker's body, for example by the rubbing of leather soles of the worker's shoes on a nylon carpet. This accumulated static charge can be discharged as a spark through the ignition material in an initiator, from the case to the firing circuit, when the initiator is touched, or when an ungrounded conductor in contact with the initiator is touched.
To prevent unintended ignition from occurring as a result of static discharge through the pyrotechnic material in the initiator, the case of the initiator may be connected electrically to the firing circuit. This allows the energy of the static charge to be dissipated in a low resistance connection, through the firing circuit, from the initiator case to the automobile body. In a coaxial device, the initiator case is normally connected to the firing circuit. In one form of twinaxial device, one of the two parallel conductors is connected electrically to the initiator case in order to provide a path for dissipation of static energy through the firing circuit. Alternatively, a spark gap may be provided between the case and one of the conductors to allow discharge of static electricity through the spark gap rather than through the pyrotechnic charge.
Preferably, in a device in which the initiator case is electrically connected to the wiring circuit, the initiator case is designed so that it does not directly contact the automobile body. For example, the case may be provided with a non-conductive cover made of plastics material. In the case of a single pole-switched firing circuit, this construction eliminates the possibility of a short circuit if the polarity of the D.C. supply to the firing circuit is accidentally reversed. It also eliminates the possibility of accidental firing, which could occur if the firing circuit supply polarity is reversed, the output leads of the firing circuit are also accidentally interchanged, and the case of the initiator touches the automobile body. Isolation of the case from the automobile body also makes it possible to use a two pole-switched firing circuit.
Constructing the device in such a way as to isolate the initiator case from the automobile body introduces the possibility of leak paths for high pressure gas generated by the propellant. Production of the plastic-covered case also requires an expensive insert molding operation.